* The reblocking code was incorrectly assuming the cursor would be pointing
at a valid node element after an unlock/relock sequence, when it could
actually be pointing at the EOF of a node. This case can occur when
the filesystem is nearly full (possibly due to the reblocking operation
itself), when the filesystem is also under load from unrelated
operations.

* This can result in the creation of a corrupted B-Tree leaf node or
data record.

* Corruption can be checked with hammer checkmap and hammer show
(as of this rev):

hammer -f device checkmap

Should output no B-Tree node records or free space mismatches.
You will still get the initial volume summary.

hammer -f device show | egrep '^B' | egrep -v '^BM'

Should output no records.

* Currently the only recourse if corruption is found is to copy off the
filesystem, newfs_hammer, and copy it back.

Full history and snapshots can be retained by using 'hammer -B mirror-read'
to copy off the filesystem and mirror-write to copy it back. However,
pleaes remember you must do this for each PFS individually. Make sure
you have a viable backup before newfsing anything.